Display device and electronic apparatus

ABSTRACT

A display device in which variations in luminance due to variations in characteristics of transistors are reduced, and image quality degradation due to variations in resistance values is prevented. The invention comprises a transistor whose channel portion is formed of an amorphous semiconductor or an organic semiconductor, a connecting wiring connected to a source electrode or a drain electrode of the transistor, a light emitting element having a laminated structure which includes a pixel electrode, an electro luminescent layer, and a counter electrode, an insulating layer surrounding an end portion of the pixel electrode, and an auxiliary wiring formed in the same layer as a gate electrode of the transistor, a connecting wiring, or the pixel electrode. Further, the connecting wiring is connected to the pixel electrode, and the auxiliary wiring is connected to the counter electrode via an opening portion provided in the insulating layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device comprising aself-light emitting element and a transistor whose channel portion isformed of an amorphous semiconductor or an organic semiconductor.

2. Description of the Related Art

In recent years, a display device comprising a light emitting elementhas been actively developed. In addition to the advantages of aconventional liquid crystal display device, the light emitting displaydevice has the features such as fast response, superior dynamic displayand wide viewing angle. Therefore, the light emitting display deviceattracts a lot of attention as a next-generation flat panel display.

The light emitting display device comprises a plurality of pixels eachhaving a light emitting element and at least two transistors. In each ofthe pixels, the transistor which is connected in series with the lightemitting element controls light emission or non-light emission of thelight emitting element. For the transistors, a polycrystallinesemiconductor (polysilicon) with high field effect mobility is used inmany cases. The light emitting element has a structure in which anelectro luminescent layer is sandwiched between a pair of electrodes.Specifically, an electro luminescent layer is formed on a patternedfirst conductive layer (a first electrode), and then a second conductivelayer (a second electrode) is formed so as to cover the whole surface ofelectro luminescent layer.

SUMMARY OF THE INVENTION

A transistor using polysilicon tends to have variations incharacteristics due to crystal defects in grain boundaries. Accordingly,the drain current of the transistor differs in each pixel even when thesame signal voltage is inputted, leading to variations in luminance.

In view of the foregoing, the invention provides a display device inwhich variations in luminance caused by variations in characteristics oftransistors are suppressed.

It is preferable that the second conductive layer (the second electrode)formed over the electro luminescent layer is heated to lower resistance.However, the electro luminescent layer has a low heat resistance and cannot withstand a high heat processing. Therefore, due to differentresistance values, a voltage applied between a pair of electrodes isdifferent between in the edges and the center of a light emittingelement, which may result in degraded image quality.

In view of the foregoing, the invention provides a display device inwhich image quality degradation due to differences in resistance valuesis prevented.

To solve the aforementioned problems, the invention takes the followingmeasures.

A display device according to the invention comprises a light emittingelement which is controlled by a transistor whose channel portion isformed of an amorphous semiconductor (typified by amorphous silicon,a-Si:H) or an organic semiconductor. Since such a transistor has fewvariations in field effect mobility and the like, it is possible tosuppress variations in luminance of the display device due to variationsin characteristics of the transistor. Further, the amorphoussemiconductor is suitable for manufacturing a large panel ranging from afew inches to a few tens of inches in size, and the manufacturingprocesses thereof are cost effective because no crystallizing step and asmall number of masks are required.

A display device according to the invention comprises an auxiliaryconductive layer (wiring) which is connected to a conductive layerformed over an electro luminescent layer. As a result, the resistance ofthe conductive layer can be lowered without heat processing, and imagequality degradation of the display device can thus be prevented. Sincethe resistance value becomes a problem as a panel is increased in size,a large panel having a size of a few tens of inches can be manufacturedvery effectively by using the invention.

A display device according to the invention comprises a substrate whichincludes a pixel portion and a driver circuit arranged at the peripheryof the pixel portion, and a driver IC which is attached on thesubstrate. The pixel portion comprises a light emitting elementincluding a light emitting material sandwiched between a pair ofelectrodes, and a plurality of transistors whose channel portions areformed of an amorphous semiconductor. The driver circuit formed on thesubstrate comprises an N-type transistor whose channel portion is formedof an amorphous semiconductor (sometimes referred to as an a-Si:HTFThereinafter), and a P-type transistor whose channel portion is formed ofan organic semiconductor (sometimes referred to as an organic TFThereinafter). The organic TFT corresponds to a transistor including alow molecular weight organic compound such as pentacene, a highmolecular weight organic compound such as PEDOT (polythiophene) and PPV(polyphenylene-vinylene), and the like. The a-Si:HTFT and the organicTFT can be formed on the same substrate as the pixel portion, and usingthis CMOS circuit as a unit circuit, a shift register, a buffer and thelike can be configured. Moreover, the driver circuit can be formed witheither N-type transistors or P-type transistors only. In such a case,the driver circuit can be formed with either the a-Si:HTFTs or theorganic TFTs only.

A display device according to the invention comprises a light emittingelement which includes a light emitting material sandwiched between afirst electrode connected to an anode line and a second electrodeconnected to a cathode line. The display device also comprises atransistor whose channel portion is formed of an amorphoussemiconductor. The display device further comprises a reverse biasvoltage applying circuit which switches potentials of the anode line andthe cathode line with each other to apply a reverse bias voltage to thelight emitting element. According to such a structure, degradation ofthe light emitting element with time can be prevented, leading to thedisplay device with an improved reliability.

A display device according to the invention comprises a light emittingelement which includes a light emitting material sandwiched between apair of electrodes, a first transistor whose gate electrode is connectedto a first power supply with a constant potential, and a secondtransistor whose gate electrode is connected to a signal line. The lightemitting element, the first transistor, and the second transistor areconnected in series between a second power supply with the samepotential as a low potential voltage and a third power supply with thesame potential as a high potential voltage. Further, each of the firsttransistor and the second transistor has a channel portion formed of anamorphous semiconductor. In such a display device, the second transistoris operated in a linear region, and thus, the amount of current flowingin the light emitting element is not affected by a slight variation inV_(GS) of the first transistor. In other words, the amount of currentflowing in the light emitting element is determined by the firsttransistor which is operated in a saturation region. Therefore,according to the invention having the aforementioned structure, it ispossible to provide a display device in which variations in luminancedue to variations in characteristics of transistors are suppressed andimage quality is improved.

By adopting the aforementioned structure, the invention can provide adisplay device in which variations in luminance due to variations incharacteristics of transistors are suppressed. Further, in a displaydevice according to the invention, the resistance of the conductivelayer can be lowered without heat processing, and image qualitydegradation can be prevented. Moreover, the display device according tothe invention comprises a transistor whose channel portion is formed ofan amorphous semiconductor, and thus a large sized and inexpensivedisplay device can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are cross sectional views showing a transistor (channelprotected type and channel etched type) using an amorphous semiconductorfor a channel portion, a light emitting element, and an auxiliary wiringconnected to one electrode of the light emitting element.

FIG. 2 is a cross sectional view showing a transistor (dual gate type)using an amorphous semiconductor for channel portion, a light emittingelement, and an auxiliary wiring connected to one electrode of the lightemitting element.

FIGS. 3A and 3B are cross sectional views showing a transistor using anamorphous semiconductor for a channel portion, a light emitting element,and an auxiliary wiring connected to one electrode of the light emittingelement.

FIGS. 4A and 4B are top plan views of a panel showing an arrangement ofan anode line, a cathode line, and an auxiliary wiring.

FIG. 5 is a view showing an opening portion and an arrangement of ananode line, a cathode line, and an auxiliary wiring.

FIG. 6 is a view showing an opening portion and an arrangement of ananode line, a cathode line, and an auxiliary wiring.

FIG. 7 is a view showing an opening portion and an arrangement of ananode line, a cathode line, and an auxiliary wiring.

FIGS. 8A and 8B are top plan views of a panel mounting a driver IC.

FIG. 9A is a top plan view of a panel mounting a linear driver IC, and9B is a perspective view of the same.

FIG. 10A shows an equivalent circuit.

FIG. 10B is across sectional view showing a CMOS circuit formed with anorganic transistor and an a-Si transistor.

FIGS. 11A to 11F are circuit diagrams of a pixel including a lightemitting element and a transistor using an amorphous semiconductor for achannel portion.

FIGS. 12A and 12B are circuit diagrams of a shift register formed withonly N-type transistors.

FIGS. 13A and 13B are timing charts showing time gray scale.

FIGS. 14A and 14B are diagrams showing a configuration of a signal linedriver circuit and a scan line driver circuit.

FIGS. 15A to 15D are views showing electronic apparatuses using theinvention.

FIGS. 16A to 16D are views showing electronic apparatuses using theinvention.

FIGS. 17A to 17D are diagrams showing a threshold compensation circuit.

FIGS. 18A to 18D are views showing a laminated structure of a lightemitting element.

FIGS. 19A and 19B are views showing a laminated structure of a lightemitting element.

FIG. 20 is a layout diagram of a pixel circuit (3 TFT/Cell).

FIG. 21 is a layout diagram of a pixel circuit (3 TFT/Cell).

FIG. 22 is a layout diagram of a pixel circuit (4 TFT/Cell).

FIG. 23 is a layout diagram of a pixel circuit (4 TFT/Cell).

DETAILED DESCRIPTION OF THE INVENTION Embodiment Mode 1

With reference to FIGS. 4A and 4B, explanation is made on an arrangementof wirings on a panel, especially an arrangement of a power supply line(hereinafter referred to as an anode line) with the same potential as ahigh potential voltage VDD, and a power supply line (hereinafterreferred to as a cathode line) with the same potential as a lowpotential voltage VSS. It is to be noted that only wirings arranged incolumns in a pixel portion 104 are shown in FIGS. 4A and 4B.

FIG. 4A is a top plan view of a panel comprising a substrate 100. On thesubstrate 100, the pixel portion 104 in which a plurality of pixels 105arranged in matrix, a signal line driver circuit 101 arranged at theperiphery of the pixel portion 104, and scan line driver circuits 102and 103 are disposed. The number of driver circuits is not exclusivelylimited, and may be changed in accordance with a configuration of thepixel 105. Further, the driver circuits are not necessarily formedintegrally on the substrate 100, and a driver IC may be attached on thesubstrate 100 by COG and the like.

A signal line 111 arranged in columns in the pixel portion 104 isconnected to the signal line driver circuit 101. Also, power supplylines 112 to 114 arranged in columns are connected to either of anodelines 107 to 109. Similarly, an auxiliary wiring 110 arranged in columnsis connected to a cathode line 106. The anode lines 107 to 109 and thecathode line 106 are lead around the pixel portion 104 and the drivercircuits arranged at the periphery of the same, and connected toterminals of an FPC.

Each of the anode lines 107 to 109 corresponds-to each of RGB. Whenapplying different potentials to each of the anode lines 107 to 109,variations in luminance between each color can be compensated. That is,a problem in that differences in the current density of an electroluminescent layer of a light emitting element in each color causevariations in luminance in each color even when the same current issupplied can be solved by using the plurality of anode lines. It is tobe noted that an electro luminescent layer is divided into colors of RGBhere, though the invention is not limited to this. When displayingmonochrome images or displaying color images by a method in whichdifferences in current density in each pixel are not to be taken inaccount, for example by using a white light emitting element incombination with a color filter, a plurality of anode lines are notrequired and a single anode line is sufficient.

FIG. 4B is a diagram showing a mask layout simply. The anode lines 107to 109 and the cathode line 106 are arranged around the signal linedriver circuit 101, and the anode lines 107 to 109 are connected to thepower supply lines 112 to 114 arranged in columns in the pixel portion104. As shown in FIG. 4B, the cathode line 106 and the auxiliary wiring110 are formed on the same conductive layer.

After forming the cathode line 106 and the auxiliary wiring 110, a firstconductive layer (a first electrode) of a light emitting element isformed, and then an insulating layer (also called a bank) is formedthereon. Subsequently, an opening portion is formed in the insulatinglayer situated on the cathode line 106 and the auxiliary wiring 110. Theopening portion exposes the cathode line 106 and the auxiliary wiring110, and an electro luminescent layer is formed at this time. Theelectro luminescent layer is selectively formed so as not to cover theopening portion situated on the cathode line 106 and the auxiliarywiring 110. Then, a second conductive layer (a second electrode) isformed so as to cover the whole electro luminescent layer, cathode line106 and auxiliary wiring 110. According to these steps, the secondconductive layer is electrically connected to the cathode line 106 andthe auxiliary wiring 110, which is one of the significant features ofthis embodiment mode. According to this feature, the resistance of thesecond conductive layer formed so as to cover the electro luminescentlayer can be lowered, and therefore, image quality degradation due tothe resistance value of the second conductive layer can be prevented.Since the resistance value becomes a problem as a panel is increased insize, such feature is quite effective in manufacturing a large panelhaving a size of a few tens of inches.

Although the second conductive layer is connected to the cathode line inthis embodiment mode, the invention is not limited to this. The secondconductive layer may be connected to the anode line, and a counterelectrode of the light emitting element is set to be an anode in thiscase.

Further, the auxiliary wiring 110 is not necessarily formed on the samelayer as the signal line arranged in columns as shown in FIGS. 4A and4B, and may be formed on the same layer as a scan line arranged in rows.An opening portion forming a contact (connection) between the auxiliarywiring 110 and the second conductive layer may be provided in columnseither in punctate or linear shapes, or in punctate and linear shapes.It may also be provided in rows either in punctate or linear shapes, orin punctate and linear shapes. Some examples of them are shownhereinafter, and the mask layout thereof is described with reference toFIGS. 5 to 7. It is to be noted that FIGS. 5 to 7 are simplified viewsin which the pixel 105 includes only a pixel electrode and the powersupply line 112 is not shown.

With reference to FIG. 5, explanation is made on a structure in whichthe auxiliary wiring 110 and the signal line 111 are formed on the sameconductive layer and the auxiliary wiring 110 is connected to the secondconductive layer via an opening portion 120 formed in linear shapes. InFIG. 5, the pixel portion 104 comprises a plurality of pixels 105arranged in matrix, as well as the signal line 111 and the auxiliarywiring 110 arranged in columns, and a scan line 128 arranged in rows.The auxiliary wiring 110 is connected to the cathode line 106. It is tobe noted that although the auxiliary wiring 110 and the cathode line 106are formed on the same conductive layer, the wiring arranged in thepixel portion 104 is referred to as the auxiliary wiring 110 whereas thewiring arranged in the other areas is referred to as the cathode line106 herein.

The linear opening portion 120 is formed over the auxiliary wiring 110and the cathode line 106. The auxiliary wiring 110 and the cathode line106 are connected to the second conductive layer via the opening portion120. In this case, the auxiliary wiring 110 is connected to the secondconductive layer via the linear opening portion 120.

With reference to FIG. 6, explanation is hereinafter made on a structurein which a linear opening portion 122 is formed over the cathode line106 and a punctate opening portion 123 is formed over the auxiliarywiring 110. This structure is different from that shown in FIG. 5 inthat the auxiliary wiring 110 is connected to the second conductivelayer via the punctate opening portion 123.

With reference to FIG. 7, explanation is hereinafter made on a structurein which an auxiliary wiring 124 and the scan line 128 are formed on thesame conductive layer and the auxiliary wiring 124 is connected to thesecond wiring via a punctate opening portion 127. In FIG. 7, the pixelportion 104 comprises a plurality of pixels 105 arranged in matrix, aswell as a signal line 111 arranged in columns, and the scan line 128 andthe auxiliary wiring 124 arranged in rows. The auxiliary wiring 124 isconnected to a cathode line 126. The auxiliary wiring 124 and thecathode line 126 are formed on different conductive layers and connectedto each other via an opening portion.

A linear opening portion 125 is formed over the cathode line 126 and apunctate opening portion 127 is formed over the auxiliary wiring 124.The cathode line 126 and the auxiliary wiring 124 are connected to thesecond conductive layer via these opening portions 125 and 127. In thiscase, the auxiliary wiring 124 is connected to the second conductivelayer via the punctate opening portion 127.

As described above, the auxiliary wiring may be formed on the sameconductive layer as a wiring arranged in columns (e.g., a signal line)as shown in FIGS. 5 and 6, or may be formed on the same conductive layeras a wiring arranged in rows (e.g., a scan line) as shown in FIG. 7.These structures do not require an additional mask and the like, andtherefore, the problem such as increase in production costs and drop inreliability can be avoided. Further, in the case where a punctateopening portion forming a contact between the auxiliary wiring and thesecond conductive layer is arranged at the edge of a pixel, reduction inthe aperture ratio can be suppressed resulting in brighter images.

With reference to FIGS. 1A to 1C, 2, and 3A and 3B, explanation is nextmade on a cross sectional structure and a mask layout of a drivingtransistor, a light emitting element, and an auxiliary wiring which areprovided over a substrate having an insulating surface.

FIG. 1C shows a mask layout of one pixel. In the pixel shown in FIG. 1C,a conductor 16 serving as a power supply line, a conductor 26 serving asa signal line, and a conductor 27 serving as an auxiliary wiring arearranged in columns, and a conductor 28 serving as a scan line isarranged in rows. The pixel further comprises a switching transistor 29and a driving transistor 30.

FIG. 1A is a cross sectional view along a line A-B-C in the mask layoutof FIG. 1C. In FIG. 1A, a gate electrode 11 is formed on a substrate 10having an insulating surface, and a gate insulating layer 12 is formedthereon. Then, an amorphous semiconductor, an N-type semiconductor, anda conductor are laminated in this order, and then patternedsimultaneously to form an amorphous semiconductor 13, N-typesemiconductors 14 and 15, and conductors 16 and 17. Subsequently,insulators 18 and 19 are formed, and a conductor 20 is formed after anopening portion is formed in a predetermined area so as to expose theconductor 17 partly. Then, a conductor 21 (first electrode, pixelelectrode), an electro luminescent layer 22, and a conductor 23 (secondelectrode, counter electrode) are formed so as to be electricallyconnected to the conductor 20. The overlapping area of the conductor 21,the electro luminescent layer 22, and the conductor 23 corresponds to alight emitting element 24. Afterwards, a protective layer 25 is formedover the whole surface.

Note that, conductors 26 and 27 are formed at the same time as theconductors 16 and 17. The conductors 26 and 27 correspond to a signalline and an auxiliary wiring respectively. By exposing the conductor 27before forming the conductor 23 (second electrode, counter electrode),the conductor 23 can be laminated on the conductor 27, thereby loweringthe resistance of the conductor 23. It is to be noted that the conductor27 serving as an auxiliary wiring is formed on the same conductive layeras the conductors 16 and 17 in the cross sectional view of FIG. 1A.

FIG. 1B shows a cross sectional structure of a driving transistor 50 andthe light emitting element 24. In FIG. 1B, the gate electrode 11 isformed on the substrate 10 having an insulating surface, and the gateinsulating layer 12 is formed thereon. After forming the amorphoussemiconductor 13, an insulator 31 serving as an etching stopper isformed. Subsequently, an N-type semiconductor and a conductor arelaminated in this order, and then patterned simultaneously to formN-type semiconductors 32 and 33 and conductors 34 and 35. Then,insulators 18, 5070, and 5080 are formed, and after an opening portionis formed in a predetermined area so as to expose the conductor 35partly, a connecting wiring 5060 formed of a conductor is formed.Afterwards, the light emitting element 24 including the conductor 21,the electro luminescent layer 22, and the conductor 23 is formed, andthen the protective layer 25 is formed.

Note that, the conductor 26 is formed at the same time as the conductors34 and 35, and a conductor 36 is formed at the same time as theconnecting wiring 5060. The conductor 26 corresponds to a signal lineand the conductor 36 corresponds to an auxiliary wiring. By exposing theconductor 36 before forming the conductor 23 (counter electrode), theconductor 23 can be laminated on the conductor 36, thereby lowering theresistance of the conductor 23. It is to be noted that the conductor 36serving as an auxiliary wiring is formed on the same layer as theconductor 20 in the cross sectional view of FIG. 1B.

FIG. 2 is a cross sectional view of a driving transistor 51 and thelight emitting element 24. In FIG. 2, the gate electrode 11 is formed onthe substrate 10 having an insulating surface, and the gate insulatinglayer 12 is formed thereon. After forming the amorphous semiconductor13, an insulator 41 serving as an etching stopper is formed, and then agate electrode 42 is formed. Subsequently, an N-type semiconductor and aconductor are laminated in this order, and patterned simultaneously toform N-type semiconductors 43 and 44 and conductors 45 and 46. Then, theinsulators 18 and 19 are formed, and after an opening portion is formedin a predetermined area so as to expose the conductor 46 partly, theconductor 20 is formed. Afterwards, the light emitting element 24including the conductor 21, the electro luminescent layer 22, and theconductor 23 is formed before forming the protective layer 25. Theconductor 36 serving as an auxiliary wiring is electrically connected tothe conductor 23.

FIG. 3A is a cross sectional view of a driving transistor 431 and alight emitting element 438. The driving transistor 431 is formed on asubstrate 430 having an insulating surface, and an insulator 440 isformed thereon. After forming an opening portion in a predeterminedarea, conductors 433 and 434 are formed on the insulator 440.Subsequently, a conductor 435 serving as a pixel electrode is formed,and then an insulator 442 is formed. After an opening portion 439 isformed in a predetermined area of the insulators 441 and 442, an electroluminescent layer 436 is formed on the insulator 442 and a conductor 437serving as a counter electrode is formed thereon. In such a manner, fourlayers of insulators are laminated in FIG. 3A.

FIG. 3B is a cross sectional view of the driving transistor 431 and alight emitting element 459. The driving transistor 431 is formed on thesubstrate 430 having an insulating surface, and then an auxiliary wiring452 and a wiring 460 electrically connected to the driving transistor431 are formed. After forming an insulator 453, an opening portion isformed in a predetermined area of the insulator 453. Subsequently, aconductor 454 serving as a pixel electrode is formed, an insulator 458is formed thereon, then, an opening portion is formed in a predeterminedarea of the insulator 458. Afterwards, electro luminescent layers 455and 456 are formed on the conductor 454, and a conductor 457 serving asa counter electrode is formed thereon. An overlapping area of theconductor 454, the electro luminescent layers 455 and 457, and theconductor 457 corresponds to the light emitting element 459.

In FIG. 3B, the electro luminescent layer 456 on the auxiliary wiring452 is formed by vapor deposition and the film thickness thereof isthin, therefore, the sides of the auxiliary wiring 452 are not coveredwith the electro luminescent layer 456. Taking advantage of thisstructure, the conductor 457 is electrically connected to the sides ofthe auxiliary wiring 452.

As shown in FIGS. 1A to 1C, 2, and 3A and 3B, the display device of theinvention comprises a light emitting element and a transistor having anamorphous semiconductor. It is preferable that the channel width W/thechannel length L of a driving transistor connected in series with thelight emitting element is set in the range of 1 to 100 (more preferably,5 to 20) in order to improve current capacity. Specifically, it isdesirable that the channel length L is in the range of 5 to 15 μm andthe channel width W is in the range of 20 to 1200 μm (more preferably 40to 600 μm). Note that, according to the aforementioned channel length Land the channel width W, a transistor occupies larger area of a pixel.Therefore, the light emitting element desirably emits light in theopposite direction of a substrate, namely, top emission.

There are three main types of transistors using an amorphoussemiconductor for a channel portion: channel etched type (FIG. 1A, andFIGS. 3A and 3B), channel protected type (FIG. 1B), and dual gate type(FIG. 2). The invention may use any of these.

One of a pair of electrodes included in the light emitting elementcorresponds to an anode, and the other corresponds to a cathode. Theanode and the cathode are preferably formed of metal, alloy, electricalconductor compound, or mixture thereof. Further, a material having ahigh work function is used for the anode whereas a material having a lowwork function is used for the cathode. An electro luminescent layer issandwiched between the anode and the cathode, and formed of at least onematerial selected from various organic materials or inorganic materials.The luminescence in the electro luminescent layer includes luminescencethat is generated when an excited singlet state returns to a groundstate (fluorescence) and luminescence that is generated when an exitedtriplet state returns to a ground state (phosphorescence).

An insulating layer may be formed of either an organic material or aninorganic material. When using an organic material, however, a barrierfilm such as a silicon nitride film is preferably provided since it hasa high hygroscopicity. Among the organic materials, a resist material isinexpensive, has a contact hole with a small diameter, and has a lowhygroscopicity as compared with other organic materials such as acryland polyimide, and thus it requires no barrier film. However, as theresist material is colored, it is preferably used for a top emissiondisplay device. Specifically, solution obtained by dissolving cresolresin and the like in solvent (propylene glycol monomethyl etheracetate; PGMEA) is coated by a spinner to form the resist material.

According to the invention adopting the aforementioned structures,variations in characteristics of transistors are reduced, and thus, itis possible to provide a display device in which variations in luminancedue to the variations in characteristics of transistors are reduced.Further, according to the invention using an amorphous semiconductor, alarge panel ranging in size from a few inches to a few tens of inchescan be effectively manufactured, because no crystallizing step and asmall number of masks are required, leading to reduction in productioncosts. In addition, depending on a heat processing temperature inmanufacturing steps, an amorphous semiconductor can be formed on aflexible substrate such as plastic, which is light, thin, andinexpensive. Therefore, an application range of the display device canbe widened.

The auxiliary wiring contributes to lower a resistance of the secondconductive layer, resulting in reduction in power consumption. Bydisposing the auxiliary wiring, defective writing and gray scale due towiring resistance can be prevented and drop in voltage can also besuppressed, thereby applying a constant voltage to the light emittingelement. Accordingly, a display device with improved image quality canbe provided. The structures described in this embodiment mode areeffective in manufacturing a large panel having a size of a few tens ofinches. This is because the resistance value becomes a problem as apanel is increased in size.

Embodiment Mode 2

An embodiment mode of the invention is described with reference todrawings.

FIG. 8A is a top plan view of a panel which includes a substrate 200having an insulating surface. On the substrate 200, a scan line drivercircuit 203 and a pixel portion 202 including a plurality of pixels 201arranged in matrix are formed. A plurality of driver ICs 205 areattached on the substrate 200, and the plurality of driver ICs 205correspond to a signal line driver circuit 204. The scan line drivercircuit 203 and the signal line driver circuit 204 are connected to apower supply circuit 206 and a controller 207.

The power supply circuit 206 supplies power to the panel, and isconnected specifically to a power supply line disposed in the pixelportion 202. The power supply line is also referred to as an anode lineor a cathode line. The anode line has the same potential as a highpotential voltage VDD and the cathode line has the same potential as alow potential voltage VSS. The controller 207 supplies a clock, a clockback, a start pulse, and a video signal to the signal line drivercircuit 204 and the scan line driver circuit 203. In the case where thesignal line driver circuit 204 includes the plurality of driver ICs 205as in this embodiment mode, the controller 207 also determines whichvideo signal is supplied to each driver IC, that is, it sorts signals.

Although only a driver circuit on the scan line side is integrallyformed on the substrate in FIG. 8A, the invention is not limited tothis, and a driver circuit on the signal line side may also beintegrally formed on the same substrate depending on the operatingfrequency of the driver circuit. However, it is preferable that thedriver circuit on the scan line side is integrally formed on thesubstrate and the driver circuit on the signal line side is formed withdriver ICs. According to this, the scan line driver circuit and thesignal line driver circuit can be operated separately, since the signalline driver circuit is operated at a frequency of 50 MHz or more (forexample 65 MHz or more), and the scan line driver circuit is operated atthe one hundredth frequency thereof, that is approximately 100 kMHz. Inthis manner, whether driver circuits are integrally formed on asubstrate or driver ICs are attached on a substrate may be selected inaccordance with an operating frequency of each driver circuit.

FIG. 8B is a top plan view of a panel which includes the substrate 200having an insulating surface. The pixel portion 202 including theplurality of pixels 201 arranged in matrix is formed on the substrate200. A driver IC 209 on a signal line side and a driver IC 208 on a scanline side are attached on the substrate 200 by COG. These driver ICs 208and 209 are connected to an external input terminal 211 with aconnecting wiring 210, and connected to the power supply circuit 206 andthe controller 207 via the external input terminal 211. The driver ICsare attached on the substrate by COG in FIG. 8B, though, the inventionis not limited to this. The driver ICs may be attached on the substrateby TAB, or connected to the substrate via an FPC instead of attachingthereon. Further, the length of long side and short side of a driver ICis not exclusively limited as well as the number of driver ICs to bemounted.

Each of the pixels 201 comprises a light emitting element including alight emitting material sandwiched between a pair of electrodes, and atransistor whose channel portion is formed of an amorphous semiconductoror an organic semiconductor. A first electrode of the light emittingelement is connected to an anode line and a second electrode thereof isconnected to a cathode line. According to the invention, potentials ofthe anode line and the cathode line are switched with each other duringa period in which a light emitting element emits no light, and thus areverse bias voltage is applied to the light emitting element. Thetiming of applying a reverse bias voltage to the light emitting elementis determined by a predetermined signal supplied from the controller 207to the power supply circuit 206. Therefore, in the invention, the powersupply circuit 206 and the controller 207 are collectively referred toas a reverse bias voltage applying circuit.

When the display device of the invention is used for displaying imageswith multi-level gray scale, time gray scale is applicable. This isbecause by applying a reverse bias voltage during a period in which alight emitting emits no light, the reverse bias voltage can be appliedwithout affecting gray scale display.

In general, either or both of the anode lines and the cathode lines inall pixels are connected in common. Therefore, a reverse bias voltagehas to be applied to all the pixels at the same time. A semiconductorelement may thus be added in order to apply a reverse bias voltage to alight emitting element. This semiconductor element corresponds to atransistor or a diode, and allows a reverse bias voltage to be appliedarbitrarily, per pixel or per line for example. Specifically, a reversebias voltage is applied to a light emitting element as soon as thesemiconductor element is turned ON. That is, when the semiconductorelement is turned ON, the light emitting element is electricallyconnected to a wiring having a lower potential than that of a counterelectrode of the light emitting element, thereby applying a reverse biasvoltage to the light emitting element. When the reverse bias voltage isapplied, the light emitting element necessarily emits no light.According to the aforementioned structure, however, a reverse biasvoltage can be applied to an arbitrary pixel at arbitrary timing,therefore, gray scale display can be performed without any problems.This structure is applicable to other driving methods for performingmulti-level gray scale such as analog driving method as well as timegray scale.

According to the invention adopting the structure described above,degradation of a light emitting element with time can be prevented,leading to a display device with an improved reliability and long lifeelements. This embodiment mode can be implemented in combination withthe aforementioned embodiment mode.

Embodiment Mode 3

In this embodiment mode, a cross sectional structure of a CMOS circuitincluding an N-type transistor whose channel portion is formed of anamorphous semiconductor and a P-type transistor whose channel portion isformed of an organic semiconductor will be described with reference toFIGS. 10A and 10B.

FIG. 10A is an equivalent circuit diagram including a P-type transistor221 and an N-type transistor 222 which are connected in series, and oneterminal of which has the same potential as VDD and the other has thesame potential as VSS. FIG. 10B is a cross sectional view of thesetransistors. In FIG. 10B, conductors 231 and 232 are formed on thesubstrate 200, and a silicon nitride 233 is formed thereon. Then, anamorphous semiconductor 234 is formed on the silicon nitride 233, andanother silicon nitride 241 is formed thereon. On the silicon nitride241, an N-type semiconductor and a conductor are laminated in thisorder, and then patterned simultaneously to form N-type semiconductors235 and 242 and electrodes 236 and 237. Subsequently, electrodes 238 and239 are formed and an organic semiconductor 240 used as a channel layeris formed thereafter. For the organic semiconductor 240, a low molecularweight organic compound such as pentacene, a high molecular weightorganic compound such as PEDOT and PPV, and the like may be used and thepentacene may be patterned by vapor deposition using a metal mask. Insuch a manner, a CMOS circuit including an N-type transistor whosechannel portion is formed of the amorphous semiconductor 234 and aP-type transistor whose channel portion is formed of the organicsemiconductor 240 is completed.

The CMOS circuit is a unit circuit of a clocked inverter and the likeforming a shift register, a buffer and the like. Therefore, the CMOScircuit may be used for a driver circuit and a pixel circuit, though theCMOS circuit of this embodiment mode is preferably used for a drivercircuit at the scan line side because of the operating frequency.Specifically, it is desirable that a driver circuit at the scan lineside is formed with the CMOS circuit of this embodiment mode and adriver circuit at the signal line side is formed with a driver IC.Although the driver circuit is formed with the CMOS circuit in thisembodiment mode, the invention is not limited to this. It is needless tosay that the driver circuit may be formed with either N-type transistors(a-Si:HTFTs) or P-type transistors (organic TFTs) only.

This embodiment mode can be implemented in combination with theaforementioned embodiment modes.

Embodiment Mode 4

The invention provides a display device comprising a plurality of pixelseach of which includes a light emitting element having a light emittingmaterial sandwiched between a pair of electrodes, and includes atransistor whose channel portion is formed of an amorphous semiconductoror an organic semiconductor. Explanation is hereinafter made on aconfiguration of the pixel with reference to FIGS. 11A to 11F.

In a pixel shown in FIG. 11A, a signal line 310 and power supply lines311 to 313 are arranged in columns, and a scan line 314 is arranged inrows. The pixel also comprises a transistor 301 for switching, atransistor 303 for driving, a transistor 304 for current controlling, acapacitor 302, and a light emitting element 305.

A pixel shown in FIG. 11C has the same configuration as that shown inFIG. 11A, except that a gate electrode of the transistor 303 isconnected to the power supply line 313 arranged in rows. That is, bothpixels in FIGS. 11A and 11C show the same equivalent circuit diagram.However, the power supply lines are formed on different conductivelayers between in the case where the power supply line 313 is arrangedin columns (FIG. 11A) and in the case where the power supply line 313 isarranged in rows (FIG. 11C). The two pixels are each shown in FIGS. 11Aand 11C in order to make a clear distinction between layers for forminga wiring connected to the gate electrode of the transistor 303 in FIG.11A and FIG. 11C.

In both FIGS. 11A and 11C, the transistors 303 and 304 are connected inseries in the pixel, and the ratio of the channel length L₃/the channelwidth W₃ of the transistor 303 to the channel length L₄/the channelwidth W₄ of the transistor 304 is set as L₃/W₃: L₄/W₄=5 to 6000:1. Forexample, when L₃, W₃, L₄, and W₄ are equal to 500 μm, 3 μm, 3 μm, and100 μm respectively, L₃/W₃:L₄/W₄ can be set 6000:1.

The transistor 303 is operated in a saturation region and controls theamount of current flowing in the light emitting element 305, whereas thetransistor 304 is operated in a linear region and controls whether acurrent is supplied to the light emitting element 305 or not. Thesetransistors 303 and 304 preferably have the same conductivity in view ofthe manufacturing step. For the transistor 303, a depletion modetransistor may be used as well as an enhancement mode transistor.According to the invention having the aforementioned structure, a slightvariation in V_(GS) of the transistor 304 does not affect the amount ofcurrent flowing in the light emitting element 305, since the transistor304 is operated in a linear region. That is, the amount of currentflowing in the light emitting element 305 is determined by thetransistor 303 operated in a saturation region. Accordingly, it ispossible to provide a display device in which variations in luminancedue to variations in characteristics of transistors are reduced andimage quality is improved.

The transistor 301 in FIGS. 11A to 11D controls a video signal input tothe pixel. When the transistor 301 is turned ON and a video signal isinputted to the pixel, the video signal is held in the capacitor 302.Although the pixel comprises the capacitor 302 in FIGS. 11A to 11D, theinvention is not limited to this. When a gate capacitor and the like canreplace the capacitor in holding a video signal, the capacitor 302 isnot necessarily provided.

The light emitting element 305 comprises an electro luminescent layersandwiched between a pair of electrodes. A pixel electrode and a counterelectrode (anode and cathode) have a potential difference in order thata forward bias voltage is applied to the light emitting element 305. Theelectro luminescent layer is formed of at least one material selectedfrom various organic materials or inorganic materials. The luminescencein the electro luminescent layer includes luminescence that is generatedwhen an excited singlet state returns to a ground state (fluorescence)and luminescence that is generated when an excited triplet state returnsto a ground state (phosphorescence).

A pixel shown in FIG. 11B has the same configuration as that shown inFIG. 11A, except that a transistor 306 and a scan line 315 are added.Similarly, a pixel shown in FIG. 11D has the same configuration as thatshown in FIG. 11C, except that the transistor 306 and the scan line 315are added.

The transistor 306 is controlled to be ON/OFF by the added scan line315. When the transistor 306 is turned ON, charges held in the capacitor302 are discharged, thereby turning the transistor 304 OFF. That is,supply of a current to the light emitting element 305 can be forciblystopped by disposing the transistor 306. Accordingly, by adopting theconfigurations shown in FIGS. 11B and 11D, a lighting period can startsimultaneously with or shortly after a writing period before signals arewritten to all the pixels, leading to increased duty ratio.

In a pixel shown in FIG. 11E, a signal line 350 is arranged in columns,and power supply lines 351 and 352 and a scan line 353 are arranged inrows. The pixel further comprises a switching transistor 341, a drivingtransistor 343, a capacitor 342, and a light emitting element 344. Apixel shown in FIG. 11F has the same configuration as that shown in FIG.11E, except that a transistor 345 and a scan line 354 are added. It isto be noted that the configuration of FIG. 11F also allows the dutyratio to be increased due to the transistor 345.

This embodiment mode can be implemented in combination with theaforementioned embodiment modes.

Embodiment 1

A light emitting element including a light emitting material between apair of electrodes and a transistor including an amorphous semiconductoror an organic semiconductor are essential elements of the invention, andthe light emitting element and the transistor are provided in eachpixel. When a transistor including an amorphous semiconductor isprovided in each pixel as in this case, a driver IC is usually mountedon a substrate by COG or TAB, or connected to a substrate via an FPC.Described hereinafter is an embodiment in which a plurality of driverICs are formed on a rectangular substrate and mounted on a substrate.

FIG. 9A is a top plan view of a panel which includes driver ICs 251 and252 on a scan line side and a signal line side, respectively. Otherelements are the same as that of the panel shown in FIG. 8B, theexplanation is therefore omitted herein.

FIG. 9B is a perspective view showing a driver IC attached on asubstrate. A plurality of driver circuits and input and output terminalsfor connecting the plurality of driver circuits are formed on asubstrate 253. When the substrate 253 is separated into stripes orrectangles using each driver circuit and corresponding input and outputterminals as a unit, a plurality of driver ICs are obtained. Then, thedriver ICs are attached on the substrate 200 to complete a displaydevice. In FIG. 9B, the driver IC 252 serving as a scan line drivercircuit and the driver IC 251 serving as a signal line driver circuitare mounted on the substrate.

It is preferable that signal lines and scan lines have the same pitch asthe output terminals of the driver ICs. According to this, it is notnecessary to provide a lead wiring for every few blocks at the end ofthe pixel portion 202, leading to improved yield in manufacturing steps.Further, by forming the driver ICs on the rectangular substrate 253,they can be produced in large quantities, leading to enhancedproductivity. Therefore, as the substrate 253, a large substrate, forexample, a substrate having a side of about 300 to 1000 mm in length ispreferably used. This provides a great advantage as compared with thecase where the IC chips are formed on a circular silicon wafer.Moreover, when the substrate 253 is separated so that the long side ofthe driver IC has the same length as the vertical or the horizontaldirection of the pixel portion 202, the number of driver ICs can bereduced and the reliability can be improved.

These driver ICs are preferably formed of a crystalline semiconductor,and the crystalline semiconductor is preferably obtained by irradiatingcontinuous wave laser light. Thus, as an oscillator generating the laserlight, either a continuous wave solid-state laser or a continuous wavegas laser is desirably used. When irradiating continuous wave laserlight, a crystal grain boundary extends in the scanning direction of thelaser light. Taking advantage of such characteristics, a semiconductorlayer is patterned so that the crystal grain boundary direction isparallel to the channel length direction. Thus, a thin film transistorusing a crystalline semiconductor having enough electricalcharacteristics as an active layer can be achieved.

It is preferable that a driver IC disposed in a signal line side andthat disposed in a scan line side have different structures, andspecifically, they are different in the thickness of a gate insulatinglayer of a thin film transistor. It is thus possible to independentlyoperate the signal (data) line driver circuit and the scan line drivercircuit. Specifically, in a thin film transistor forming the signal linedriver circuit, the thickness of a gate insulating layer is set 20 to 70nm and the channel length is set 0.3 to 1 μm. On the other hand, in athin film transistor forming the scan line driver circuit, the thicknessof a gate insulating layer is set 150 to 250 nm and the channel lengthis set 1 to 2 μm. In such a manner, a display device comprising driverICs each of which has an operating frequency corresponding to eachdriver circuit can be achieved. This embodiment mode can be implementedin combination with the aforementioned embodiment modes.

Embodiment 2

A light emitting element including a light emitting material between apair of electrodes and a transistor including an amorphous semiconductoror an organic semiconductor are essential elements of the invention, andthe light emitting element and the transistor are provided in eachpixel. In such a transistor including an amorphous semiconductor,electrical characteristics (threshold voltage, field effect mobility andthe like) are varied with time. Thus, a threshold compensation circuitis hereinafter described, referring to a threshold voltage.

A threshold compensation circuit is explained with reference to FIGS.17A to 17D. FIG. 17A is an equivalent circuit which includes switches531 and 532 formed with transistors and the like, a transistor 533, anda capacitor 534. The operation of this circuit is briefly described.

When the switches 531 and 532 are turned ON (FIG. 17A), a current I_(ds)is supplied from the switch 531 to the transistor 533 and from theswitches 531 and 532 to the capacitor 534. The I_(ds) is divided into I₁and I₂, and I_(ds)=I₁+I₂ is satisfied. When the current starts flowing,charges are not held in the capacitor 534, the transistor is turned OFF,and thus, I₂=0 and I_(ds)=I₁ are satisfied. However, as the charges areheld in the capacitor 534, potentials between two electrodes of thecapacitor 534 starts differing. When potential difference between theelectrodes is equal to V_(th), a transistor 533 is turned ON, and I₂ ismore than 0. Since I_(ds)=I₁+I₂ is satisfied at this time, I₁ isdecreased gradually, though the current continues to flow. The capacitor534 continues to hold charges until the potential difference between theelectrodes is equal to V_(dd). When the potential difference between theelectrodes is equal to V_(dd), I₂ stops flowing, and as the transistor533 is turned ON, I_(ds)=I₁ is satisfied (FIGS. 17C and 17D, point A).

Subsequently, the switch 531 is turned OFF (FIG. 17B). Thus, the chargesheld in the capacitor 534 flow in the direction of the transistor 533via the switch 532 to be discharged. This operation continues until thetransistor 533 is turned OFF. That is, the capacitor 534 holds chargeshaving the same potential as a threshold voltage of the transistor 533(FIGS. 17C and 17D, point B).

In this manner, potential difference between two electrodes of acapacitor can be set the same as a threshold voltage of a transistor. Asignal voltage is inputted to a gate electrode of the transistor whileholding V_(GS) of the transistor. Thus, the V_(GS) held in the capacitoradded to the signal voltage is inputted to the gate electrode of thetransistor. In other words, even when there are variations in thresholdvoltages of transistors, a signal voltage and a threshold voltage of atransistor are constantly inputted to the transistor. Therefore,variations in threshold voltages of transistors can be reduced.

By using the threshold compensation circuit, variations in thresholdvoltages of driving transistors for driving a light emitting element canbe reduced, variations in luminance due to such variations in thresholdvoltages can also be reduced, and a display device with improved imagequality can be achieved. It is to be noted that the thresholdcompensation circuit of this embodiment mode can be applied to the pixelcircuits shown in FIGS. 11A to 11F. In this case, the thresholdcompensation circuit may be provided so as to compensate a thresholdvoltage of a driving transistor having a gate electrode to which asignal voltage is inputted.

Although a compensator for a threshold voltage is shown as an example inthis embodiment, the invention may comprise a compensator for otherelectrical characteristics. For example, a compensator for field effectmobility may be provided. This embodiment can be implemented incombination with the aforementioned embodiment modes and embodiment.

Embodiment 3

In order to form a light emitting element, a hole injecting layer, ahole transporting layer, a hole blocking layer, an electron transportinglayer and the like are arbitrarily combined. Though, an electroninjecting layer is preferably formed of bathocuproine (BCP) known as amaterial suitable for transporting only electrons, which is doped withlithium (Li), since electron injection property can be drasticallyimproved when bathocuproine is doped with lithium.

Further, benzoxazole derivative (BzOS) and pyridine derivative arematerials which have excellent electron transport property and are notcrystallized easily when deposited. In addition, these materials canhave excellent electron injection property when containing at least oneof alkaline metal, alkaline earth metal, and transition metal.Therefore, in a light emitting element comprising a light emittingmaterial between a pair of electrodes, a part of layers included in thelight emitting material is preferably formed of benzoxazole derivativeor pyridine derivative.

That is, when an electron injecting layer is formed of an electroninjection property composition for light emitting element includingeither benzoxazole derivative or pyridine derivative and at least one ofalkaline metal, alkaline earth metal, and transition metal, electronscan be injected more easily from an electrode functioning as a cathode.Moreover, as pyridine derivative is not efficiently crystallized whendeposited, a light emitting element having superior characteristics andlonger life than ever before can be provided as well as a display deviceusing the same. This embodiment can be implemented in combination withthe aforementioned embodiment modes and embodiments.

Embodiment 4

In this embodiment, a laminated structure of a light emitting element isdescribed. It is to be noted that the description is performed hereinwith reference to enlarged views of areas 5700 and 5710 surrounded by adotted line in FIG. 1B. FIGS. 18A, 18C, and 19A correspond to enlargedviews of the area 5700, and FIGS. 18B, 18D, and 19B correspond toenlarged views of the area 5710. A cross sectional structure of FIG. 1Band cross sectional structures of FIGS. 18A to 18D and FIGS. 19A and 19Bare the same in that they comprise the insulators 5070 and 5080, theconnecting wiring 5060, and a pixel electrode 5100, but they aredifferent in other elements which will be described hereinafter by theuse of different reference numerals.

In FIGS. 18A and 18B, the insulator 5080 is formed on the insulator5070, and the connecting wiring 5060 is formed thereon. The connectingwiring 5060 is electrically connected to either a source electrode or adrain electrode of a driving transistor. On the insulator 5080, anauxiliary wiring 5200 obtained by patterning the same conductor as theconnecting wiring 5060 is also formed. Then, the pixel electrode 5100 isformed so as to be connected with the connecting wiring 5060, and on thepixel electrode 5100, a hole injecting layer 5110, a light emittinglayer 5120, and an electron injecting layer 5130 are laminated in thisorder. Finally, a protective layer 5240 is formed. An overlapping areaof the pixel electrode 5100, the hole injecting layer 5110, the lightemitting layer 5120, and the electron injecting layer 5130 correspondsto a light emitting element 5140.

The light emitting layer 5120 is formed by using a metal mask so as toexpose a part of the auxiliary wiring 5200 while not covering an openingportion entirely. Accordingly, in the opening portion, the holeinjecting layer 5110 and the electron injecting layer 5130 are laminatedin this order on the auxiliary wiring 5200. Note that, the invention isnot limited to this structure, and only the electron injecting layer5130 may be on the auxiliary wiring 5200 by forming the hole injectinglayer 5110 and the light emitting layer 5120 by means of a metal mask.

Although light from the light emitting element 5140 is emitted in thedirection of the substrate in FIGS. 18A and 18B, the structure in whichthe light is emitted in the opposite direction of the substrate may alsobe adopted.

In FIGS. 18C and 18D, the pixel electrode 5100 is formed so as to beconnected with the connecting wiring 5060. On the pixel electrode 5100,the hole injecting layer 5110, the light emitting layer 5120, theelectron injecting layer 5130, and a transparent conductive layer 5800are laminated in this order. Finally, the protective layer 5240 isformed. The transparent conductive layer 5800 formed so as to be inconnect with the electron injecting layer 5130 suppresses drop involtage even when the electron injecting layer 5130 serving as a counterelectrode has increased resistance.

The light emitting layer 5120 is formed by means of a metal mask so asto expose a part of the auxiliary wiring 5200 while not covering anopening portion entirely. Accordingly, in the opening portion, the holeinjecting layer 5110, the electron injecting layer 5130, and thetransparent conductive layer 5800 are laminated in this order on theauxiliary wiring 5200. It is to be noted that the invention is notlimited to this structure, and only the electron injecting layer 5130and the transparent conductive layer 5800 may be formed on the auxiliarywiring 5200 by forming the hole injecting layer 5110 and the lightemitting layer 5120 by means of a metal mask. Alternatively, only thetransparent conductive layer 5800 may be formed on the auxiliary wiring5200 by forming the hole injecting layer 5110, the light emitting layer5120, and the electron injecting layer 5130 by means of a metal mask.

The protective layer 5240 shown in FIGS. 18A and 18B may have alaminated structure of an inorganic insulating layer and an organicinsulating layer. A cross sectional structure in this case is describedwith reference to FIGS. 19A and 19B.

In FIGS. 19A and 19B, the protective layer 5240 has a laminatedstructure in which an inorganic insulating layer 5240 a is formed so asto be in contact with the electron injecting layer 5130, and an organicresin layer 5240 b and an inorganic insulating layer 5240 c arelaminated in this order on the inorganic insulating layer 5240 a. Whenthe inorganic insulating layers 5240 a and 5240 c are formed of siliconnitride, silicon nitride oxide, aluminum oxide, aluminum nitride or thelike, moisture and oxygen can be prevented from being absorbed in thelight emitting element 5140 and accelerating the degradation thereof.Further, the organic resin layer 5240 b with less internal stressprovided between the inorganic insulating layer 5240 a and the inorganicinsulating layer 5240 c can prevent the protective layer 5240 from beingpeeled off by stress. For the organic resin layer 5240 b, polyimide,polyamide, polyimide amide or the like may be used.

The light emitting layer 5120 is formed by using a metal mask so as toexpose a part of the auxiliary wiring 5200 while not covering an openingportion entirely. Accordingly, in the opening portion, the holeinjecting layer 5110, the electron injecting layer 5130, the inorganicinsulating layer 5240 a, the organic resin layer 5240 b, and theinorganic insulating layer 5240 c are laminated in this order on theauxiliary wiring 5200. It is to be noted that the invention is notlimited to this structure, and by forming the hole injecting layer 5110and the light emitting layer 5120 by means of a metal mask, the electroninjecting layer 5130, the inorganic insulating layer 5240 a, the organicresin layer 5240 b, and the inorganic insulating layer 5240 c may belaminated in this order on the auxiliary wiring 5200. This embodimentcan be implemented in combination with the aforementioned embodimentmodes and embodiments.

Embodiment 5

A light emitting element including a light-emitting material between apair of electrodes and a transistor including an amorphous semiconductoror an organic semiconductor are essential elements of the invention, andthe light emitting element and the transistor are provided in eachpixel. In the case of providing such a transistor in each pixel, adriver circuit formed on the same substrate is also preferably formedwith transistors including an amorphous semiconductor or an organicsemiconductor. However, a transistor including an amorphoussemiconductor can not be applied to a P-type transistor. In thisembodiment, a shift register formed only with N-type transistors willthus be described.

In FIG. 12A, a block denoted by 400 corresponds to a pulse outputcircuit for outputting sampling pulses of one stage. A shift register isformed with n pulse output circuits. FIG. 12B shows a specificconfiguration of the pulse output circuit 400, which includes N-typetransistors 401 to 406 and a capacitor 407. The pulse output circuit 400can be made only with the N-type transistors by applying the bootstrapmethod. The operation is disclosed in detail in Japanese PatentLaid-Open No. 2002-335153.

Although the driver circuit is made only with N-type transistors in thisembodiment, the invention is not limited to this. A P-type transistorwhose channel portion includes an organic semiconductor may be used forforming the driver circuit. This embodiment can be implemented incombination with the aforementioned embodiment modes and embodiments.

Embodiment 6

In the case where the display device of the invention is operated bydigital driving method, time gray scale is preferably used fordisplaying images with multi-level gray scale. In this embodiment, thetime gray scale is described. FIG. 13A is a timing chart whose ordinaterepresents scan lines and abscissa represents time. FIG. 13B is a timingchart of a scan line of j-th row.

The display device has a frame frequency of approximately 60 Hz herein.Namely, writing of image is performed 60 times per second, and a periodof one writing image is referred to as a frame period. In the time grayscale, a frame period is divided into a plurality of subframe periods.The number of divisions is equal to the number of bits in many cases,and such a case is described herein for simplicity. That is, as 5-bitgray scale is shown as an example in this embodiment, a frame period isdivided into five subframe periods SF1 to SF5. Each subframe periodcomprises an address period Ta for writing a video signal to a pixel,and a sustain period Ts for lighting or non-lighting of the pixel. Theratio of the sustain periods Ts1 to Ts5 is set as Ts1: . . .:Ts5=16:8:4:2:1. In other words, when displaying an image with n-bitgray scale, the ratio of the sustain periods is 2^((n-1)):2^((n-2)): . .. :2¹:2⁰.

A subframe period having a shorter lighting period than a writing period(the subframe period SF5 herein) has an erasing period Te5. During theerasing period Te5, a video signal which has been written to a pixel isreset and a light emitting element is forcibly reset in order that thenext period does not start shortly after a lighting period.

When the number of bits has to be increased, the number of subframes maybe increased. The order of subframe periods is not necessarily arrangedfrom the most significant bit to the least significant bit, and it maybe arranged at random in a frame period. Further, the order of subframeperiods may be changed per frame period. This embodiment can beimplemented in combination with the aforementioned embodiment modes andembodiments.

Embodiment 7

In this embodiment, a configuration example of a signal line drivercircuit and a scan line driver circuit is described with reference toFIGS. 14A and 14B.

As shown in FIG. 14A, a signal line driver circuit comprises a shiftregister 3021, a first latch circuit 3022, and a second latch circuit3023. Meanwhile, as shown in FIG. 14B, a scan line driver circuitcomprises a shift register 3024 and a buffer 3025. The configurations inFIGS. 14A and 14B are just examples. For example, a level shifter or abuffer may be added to the signal line driver circuit, and a levelshifter may be disposed between the shift register 3024 and the buffer3025 in the scan line driver circuit. By adding the level shifter,voltage amplitude of a logic circuit portion and a buffer portion can bechanged. This embodiment can be implemented in combination with theaforementioned embodiment modes and embodiments.

Embodiment 8

The invention can be applied to various electronic apparatuses such as avideo camera, a digital camera, a goggle type display, a navigationsystem, an audio reproducing device such as a car audio system, anotebook personal computer, a game machine, a portable informationterminal (a mobile computer, a mobile phone, a portable game machine, anelectronic book and the like), an image reproducing device provided witha recording medium, such as a home video game machine (specifically, adevice which is capable of reproducing a recording medium such as a DVDand has a display for displaying the reproduced image). The specificexamples of such electronic apparatuses are shown in FIGS. 15A to 15Dand 16A to 16D.

FIG. 15A shows a portable terminal which includes a main body 9301, anaudio output portion 9302, an audio input portion 9303, a displayportion 9304, an operation switch 9305 and the like. FIG. 15B shows aPDA which includes a main body 9101, a stylus 9102, a display portion9103, operation keys 9104, an external interface 9105 and the like. FIG.15C shows a portable game machine which includes a main body 9201, adisplay portion 9202, operation keys 9203 and the like. FIG. 15D shows agoggle type display which includes a main body 9501, a display portion9502, an arm portion 9503 and the like.

FIG. 16A shows a large liquid crystal television having a size of about40 inches, which includes a display portion 9401, a housing 9402, anaudio output portion 9403 and the like. FIG. 16B shows a monitor whichincludes a housing 9601, an audio output portion 9602, a display portion9603 and the like. FIG. 16C shows a digital camera which includesdisplay portions 9701 and 9702 and the like. FIG. 16D shows a notebookpersonal computer which includes a housing 9801, a display portion 9802,a keyboard 9803 and the like.

In the aforementioned electronic apparatuses, the display device of theinvention can be applied to a panel including the display portions 9304,9103, 9202, 9502, 9401, 9603, 9701, 9702, and 9802. This embodiment canbe implemented in combination with the aforementioned embodiment modesand embodiments.

Embodiment 9

In this embodiment, a layout example of the pixel circuit described inEmbodiment 4 is explained. A layout example described hereinafterincludes a pixel electrode of a light emitting element and an insulatinglayer surrounding an end portion of the pixel electrode. In layoutexamples shown in FIGS. 21 to 23, three pixels adjacent to each otherare shown, and one of the pixels shows a layout shortly after forming atransistor and a capacitor, another pixel shows a layout shortly afterforming a pixel electrode, and the rest of the pixels shows a layoutshortly after forming an insulating layer serving as a bank.

The first and second layout examples show a pixel having threetransistors (3 TFT/Cell). The pixel comprises a switching transistor601, a driving transistor 602, an erasing transistor 603, a capacitor604, a signal line 609 and an auxiliary wiring 610 arranged in columns,and scan lines 607 and 608 arranged in rows (FIGS. 20 and 21). The pixelalso comprises a pixel electrode 605 included in a light emittingelement and an insulating layer 606. The insulating layer 606 isprovided between the adjacent pixel electrodes 605, and has an openingportion so as to expose the auxiliary wiring 610 and the pixel electrode605. The auxiliary wiring 610 is connected to a counter electrode viathe opening portion provided in the insulating layer 606. An electroluminescent layer is provided so as to be in contact with the pixelelectrode 605 via the opening portion in the insulating layer 606, andthe counter electrode is provided so as to be in contact with theelectro luminescent layer.

In the layout example shown in FIG. 20, either top emission, bottomemission, or dual emission may be adopted. On the other hand, in thelayout example shown in FIG. 21, top emission is preferably adoptedsince the pixel electrode 605 is provided over the transistors 601 to603. Dual emission may also be adopted in FIG. 21. In that case, theinsulating layer 606 may be formed of a shielding material in order toshade the transistors 601 to 603.

The third and fourth layout examples show a pixel having fourtransistors (4 TFT/Cell). The pixel comprises a transistor 611 forswitching, a transistor 619 for driving, a transistor 620 for currentcontrolling, a transistor 613 for erasing, a capacitor 614, a signalline 612 and an auxiliary wiring 621 arranged in columns, and scan lines617 and 618 arranged in rows (FIGS. 22 and 23). The pixel also comprisesa pixel electrode 615 included in a light emitting element and aninsulating layer 616. The insulating layer 616 has an opening portion soas to expose the auxiliary wiring 621 and the pixel electrode 615. Theauxiliary wiring 621 is connected to a counter electrode via the openingportion provided in the insulating layer 616. Further, an electroluminescent layer is provided so as to be in contact with the pixelelectrode 615 via the opening portion in the insulating layer 616, andthe counter electrode is provided so as to be in contact with theelectro luminescent layer. According to such a structure, the pixelelectrode 615 is provided over the transistors 611, 613, 619, and 620leading to improved aperture ratio. Therefore, top emission ispreferably adopted in this structure. It is to be noted that in thelayout example shown in FIG. 22, dual emission may also be adopted. Inthat case, the insulating layer 616 may be formed of a shieldingmaterial so as to shade the transistors 611, 613, 619, and 620.

In the structures described above, the transistors 601 to 603, 611, 613,619, and 620 include an amorphous semiconductor or an organicsemiconductor for the channel portion. The auxiliary wiring 610 and 621are formed either in the same layer as a gate electrode of thetransistors 601 to 603, 611, 613, 619, and 620, in the same layer as aconnecting wiring connected to either a source electrode or a drainelectrode of the transistors 601 to 603, 611, 613, 619, and 620, or inthe same layer as the pixel electrodes 605 and 615.

This application is based on Japanese Patent Application serial no.2003-172009 filed in Japan Patent Office on 17 Jun. 2003, the contentsof which are hereby incorporated by reference.

Although the present invention has been fully described by way ofEmbodiment Modes and Embodiments with reference to the accompanyingdrawings, it is to be understood that various changes and modificationswill be apparent to those skilled in the art. Therefore, unless suchchanges and modifications depart from the scope of the present inventionhereinafter defined, they should be constructed as being includedtherein.

1. A display device comprising: a gate electrode provided over asubstrate; a gate insulating layer provided over the gate electrode; afirst semiconductor region provided over the gate insulating layer andhaving one conductivity; a second semiconductor region provided over thegate insulating layer and having the one conductivity; a channel regionprovided over the gate insulating layer and between the firstsemiconductor region and the second semiconductor region and comprisingan amorphous semiconductor; a connecting wiring connected with one ofthe first semiconductor region and the second semiconductor region; anauxiliary wiring provided over the gate insulating layer and in a samelayer as the connecting wiring; a first insulating layer provided overthe auxiliary wiring, the first semiconductor region, the secondsemiconductor region and the connecting wiring; a pixel electrodeprovided over the first insulating layer and connected with theconnecting wiring; a second insulating layer provided over the firstinsulating layer; a first opening portion provided over the pixelelectrode and in the second insulating layer; a second opening portionprovided over the auxiliary wiring and in the first insulating layer andthe second insulating layer; an electroluminescent layer provided incontact with the pixel electrode via the first opening portion with thesecond insulating layer provided over an end portion of the pixelelectrode; and a counter electrode provided over the electroluminescentlayer, wherein the auxiliary wiring is connected with the counterelectrode via the second opening portion, and wherein the counterelectrode extends into the second opening portion to provide the counterelectrode over the auxiliary wiring.
 2. A display device according toclaim 1 wherein the substrate comprises a flexible substrate.
 3. Adisplay device according to claim 1 wherein the display device isincorporated into one selected from the group consisting of a portableterminal, a portable game machine, a goggle type display, a monitor, acamera and a personal computer.
 4. A display device according to claim 1further comprising a protective layer provided over the counterelectrode.
 5. A display device comprising: a gate electrode providedover a substrate; a gate insulating layer provided over the gateelectrode; a first N-type semiconductor region provided over the gateinsulating layer; a second N-type semiconductor region provided over thegate insulating layer; a channel region provided over the gateinsulating layer and between the first N-type semiconductor region andthe second N-type semiconductor region and comprising an amorphoussemiconductor; a connecting wiring connected with one of the firstN-type semiconductor region and the second N-type semiconductor region;an auxiliary wiring provided over the gate insulating layer and in asame layer as the connecting wiring; a first insulating layer providedover the auxiliary wiring, the first N-type semiconductor region, thesecond N-type semiconductor region and the connecting wiring; a pixelelectrode provided over the first insulating layer and connected withthe connecting wiring; a second insulating layer provided over the firstinsulating layer; a first opening portion provided over the pixelelectrode and in the second insulating layer; a second opening portionprovided over the auxiliary wiring and in the first insulating layer andthe second insulating layer; an electroluminescent layer provided incontact with the pixel electrode via the first opening portion with thesecond insulating layer provided over an end portion of the pixelelectrode; and a counter electrode provided over the electroluminescentlayer, wherein the auxiliary wiring is connected with the counterelectrode via the second opening portion, and wherein the counterelectrode extends into the second opening portion to provide the counterelectrode over the auxiliary wiring.
 6. A display device according toclaim 5 wherein the substrate comprises a flexible substrate.
 7. Adisplay device according to claim 5 wherein the display device isincorporated into one selected from the group consisting of a portableterminal, a portable game machine, a goggle type display, a monitor, acamera and a personal computer.
 8. A display device according to claim 5further comprising a protective layer provided over the counterelectrode.
 9. A display device comprising: a gate electrode providedover a substrate; a gate insulating layer provided over the gateelectrode; a first semiconductor region provided over the gateinsulating layer and having one conductivity; a second semiconductorregion provided over the gate insulating layer and having the oneconductivity; a channel region provided over the gate insulating layerand between the first semiconductor region and the second semiconductorregion and comprising an amorphous semiconductor; a connecting wiringconnected with one of the first semiconductor region and the secondsemiconductor region; an auxiliary wiring provided over the gateinsulating layer and in a same layer as the connecting wiring; a firstinsulating layer provided over the auxiliary wiring, the firstsemiconductor region, the second semiconductor region and the connectingwiring; a pixel electrode provided over the first insulating layer andconnected with the connecting wiring; a second insulating layer providedover the first insulating layer; a first opening portion provided overthe pixel electrode and in the second insulating layer; a second openingportion provided over the auxiliary wiring and in the first insulatinglayer and the second insulating layer; an electroluminescent layerprovided in contact with the pixel electrode via the first openingportion with the second insulating layer provided over an end portion ofthe pixel electrode; and a counter electrode provided over theelectroluminescent layer and the second insulating layer, wherein theauxiliary wiring is connected with the counter electrode via the secondopening portion, and wherein the counter electrode extends into thesecond opening portion to provide the counter electrode over theauxiliary wiring.
 10. A display device according to claim 9 wherein thesubstrate comprises a flexible substrate.
 11. A display device accordingto claim 9 wherein the display device is incorporated into one selectedfrom the group consisting of a portable terminal, a portable gamemachine, a goggle type display, a monitor, a camera and a personalcomputer.
 12. A display device according to claim 9 further comprising aprotective layer provided over the counter electrode.